Bandpass filer having parallel-coupled lines

ABSTRACT

A bandpass filter having parallel-coupled lines is disclosed having at least one pair of two-terminal parallel-coupled lines disposed consecutively in a step form, wherein the widths of the pairs of two-terminal parallel couple lines are alternately increased and decreased.

BACKGROUND OF THE INVENTION

The present invention pertains to a bandpass filter for use in the super-high-frequency (SHF) band, and particularly to a bandpass filter having parallel-coupled lines which uses microstrip lines as a resonator.

Generally, a bandpass filter for use in the SHF band is employed for the output port of an SHF transmitter, the input port of an SHF receiver and the output port of a frequency converter, so as to reduce the insertion loss of a transmitted signal and to enhance the capability of removing unwanted frequencies. Such a bandpass filter is utilized in an amplifier and frequency converter required for the configuration of ground microwave systems and satellite communication systems. SHF bandpass filters have been recently constructed such that an array of microstrip lines are formed in parallel. However, in the bandpass filter having parallel-coupled lines and using microstrip lines, the distance between parallel-coupled lines of the first and last parallel microstrip lines is below a specific value (0.1 mm), which makes the manufacturing of the filter difficult. Therefore, during filter design, the precise estimation of the insertion loss and bandwidth of such a bandpass filter is difficult.

Such problems will be described below in detail with reference to the attached drawings.

FIG. 1 is a schematic view of a general four-terminal parallel-coupled transmission line.

Referring to FIG. 1, the parallel-coupled transmission line comprises terminals 1 and 4 which constitute an input port, terminals 2 and 3 which constitute an output port, and microstrip lines 5 and 6 disposed in parallel while being spaced apart by a distance d and each characterized by having a length l and a width W. Here, length l has a value corresponding to one fourth the wavelength (λ/4) of a signal.

FIG. 2 is a schematic view of a conventional bandpass filter having parallel-coupled lines and using a stepped impedance resonator. Referring to FIG. 2, two-terminal parallel-coupled lines BL₁ ˜BL_(n+1) (wherein terminals 3 and 4 of the four-terminal parallel-coupled line of FIG. 1 are left open) are consecutively arranged in a step form. The two-terminal parallel-coupled lines BL₁ ˜BL_(n+1) are formed with microstrip lines SL₁ ˜SL_(2n+2) which are disposed so as to have different distances d₁ ˜d_(n+1). Impedance Z₀ indicates the characteristic impedance of the input line and output line.

FIG. 3A is an equivalent circuit diagram of an arbitrary (i+1)th two-terminal parallel-coupled line BL_(i+1) of the bandpass filter having parallel-coupled lines shown in FIG. 2. Referring to FIG. 3A, for admittance inverter j.sub.(i,i+1), the characteristic impedance thereof equals that of the input/output lines of the bandpass filter. Also, input/output lines φL_(2i) and φL_(2i+1) are each one quarter wave in length.

FIG. 3B is an equivalent circuit diagram of the bandpass filter having parallel-coupled lines shown in FIG. 2. Referring to FIG. 3B, n+1 admittance inverters j.sub.(0,1) ˜j.sub.(n,n+1) are connected in series via input/output lines φL₀ ˜φL_(2n+1) each of which are also one quarter wave in length. The characteristic impedance of the quarter-wavelength input/output lines φL₀ ˜φL_(2n+1) is equal to input/output impedance Z₀ of the bandpass filter. Therefore, the impedances Z(e)₀ (even mode) and Z(o)₀ (odd mode) of each of the two-terminal parallel-coupled lines BL₁ ˜BL_(n+1) shown in FIG. 2 are expressed as follows:

    Z(e).sub.0(i,i+1) =Z.sub.0 [1+Z.sub.0 ·j.sub.(i,i+1) {Z.sub.0 ·j.sub.(i,i+1) }.sup.2 ]                         (1)

    Z(o).sub.0(i,i+1) =Z.sub.0 [1+Z.sub.0 ·j.sub.(i,i+1) -{Z.sub.0 ·j.sub.(i,i+1) }.sup.2 ]                         (2)

Using Equations (1) and (2), if the impedances of the even mode and odd mode of the first and last parallel-coupled lines BL₁ and BL_(n+1) of the bandpass filter shown in FIG. 2 are calculated, it is noted that the impedances Z(e)₀(0,1) and Z(o)₀(0,1) of the first parallel-coupled line is the same as the impedances Z(e)₀(n,n+1) and Z(O)₀(n,n+1) of the last parallel-coupled line. Using the impedance values of the even mode and odd mode, if the width W of microstrip lines SL₁, SL₂, SL_(2n+1) and SL_(2n+2) constituting the first and last parallel-coupled lines BL₁ and BL_(n+1) and distance d between the microstrip lines are calculated, the value of distance d is less than 0.1 mm. This is not easy to accomplish with ordinary print circuit boards (for instance, epoxy-glass boards).

To overcome the above problem (when d<0.1 mm), Mitsuo Makimoto and Sadahiko Yamashita (both of Japan) have disclosed in a paper entitled "Strip-line Resonator Filters having Multi-coupled Sections (IEEE MTT-S DIGEST, pp. 92-94, 1983), that the first and last terminal pairs BL₁ and BL_(n+1) of the bandpass filter shown in FIG. 2 are multi-coupled, as shown in FIG. 4. However, in a filter having such a structure, the microstrip lines are discontinuous in the multi-coupled portions 30 and 31, which causes errors in circuit interpretation and thus impedes the precise estimation of insertion loss and bandwidth of a specific bandpass filter during design.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a bandpass filter having parallel-coupled lines and using a stepped impedance resonator which increases the distance between microstrip lines for the first and last parallel-coupled microstrip lines of the filter, and which has no discontinuous sections in the microstrip lines.

To accomplish the object of the present invention, there is provided a bandpass filter having parallel-coupled lines, comprising at least one pair of two-terminal parallel-coupled lines disposed consecutively in a step form, wherein the width of the at least pair of two-terminal parallel couple lines is alternately increased and decreased.

BRIEF DESCRIPTION OF THE DRAWINGS

The above object and other advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which:

FIG. 1 is a schematic view of a four-terminal parallel-coupled transmission line;

FIG. 2 is a schematic view of a conventional bandpass filter having parallel-coupled lines;

FIG. 3A is an equivalent circuit diagram of an (i+1)th two-ports parallel-coupled line of the bandpass filter shown in FIG. 2;

FIG. 3B is an equivalent circuit diagram of the bandpass filter shown in FIG. 2;

FIG. 4 shows a multi-coupled structure of the first and last terminal pairs of the parallel-coupled lines of the bandpass filter shown in FIG. 2;

FIG. 5 is a schematic view of a bandpass filter of the present invention;

FIG. 6 is an equivalent circuit diagram of the bandpass filter having parallel-coupled lines shown in FIG. 5; and

FIG. 7 is a characteristic graph of the insertion loss and return loss of the bandpass filter having parallel-coupled lines shown in FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 5, n+1 two-terminal parallel-coupled lines BL₁ ˜BL_(n+1) are consecutively arrayed in a step form. To form n+1 parallel-coupled lines BL₁ ˜BL_(n+1), microstrip lines SL₁ ˜SL_(n+2) each disposed in parallel and spaced apart by a predetermined distance d₁ ˜d_(n+1) (same as di+1) have widths W_(i+1) (where i is an even number including zero) which are much wider or much narrower than those of adjacent pairs of the microstrip lines of parallel-coupled lines BL₁ ˜BL_(n+1), and have lengths corresponding to one fourth the wavelength (λ/4) of the signal to be processed. Each microstrip line consists of a first portion having a first width and a second portion having a second width, with the first portion being the narrower width and the second portion being wider. The first parallel-coupled line BL₁ is composed of the first portion of microstrip line SL₁ of width W₁ and the first portion of microstrip line SL₂ of width W₁, and the distance between the microstrip lines is d₁. The second parallel-coupled line BL₂ is composed of the second portion of microstrip line SL₂ of width W₂ and the second portion of microstrip line SL₃ of width W₂, and the distance between the microstrip lines is d₂. The central parallel-coupled line BL_(n/2+1) is composed of the second portion of microstrip line SL_(n/2+1) of width W_(n/2+1) and the second portion of microstrip line SL_(n/2+2) of width W_(n/2+1), and the distance between the microstrip lines is d_(n/2+1). The parallel-coupled lines are symmetric with respect to the central parallel-coupled line BL_(n/2+1) so that width W₁ and distance d₁ of the first parallel-coupled line BL₁ is the same as those of the (n+1)th parallel-coupled line BL_(n+1). Similarly, width W₂ and distance d₂ of the second parallel-coupled line BL₂ is the same as those of the nth parallel-coupled line BL_(n). Microstrip line SL₁ functions as the input port of the bandpass filter and microstrip line SL_(n+2) acts as the output port thereof.

Referring to FIG. 6, n+1 admittance inverters j.sub.(0,1) ˜j.sub.(n,n+1) have quarter-wavelength input/output lines φL₀ ˜φL_(2n+1) on the input and output port sides. The characteristic impedance of quarter-wavelength input/output lines φL₀ ˜φL_(2n+1) of each of the admittance inverters j.sub.(0.1) ˜j.sub.(n,n+1) is set as Z₀(i+1) which is a different value from the characteristic impedances of the quarter-wavelength lines of the adjacent admittance inverters.

According to the characteristic impedance Z₀(i+1) of the quarter-wavelength input/output lines φL₁ ˜φL_(2n+1) shown in FIG. 6, the impedances Z(e)₀ (even mode) and Z(o)₀ (odd mode) of the two-terminal parallel-coupled lines BL₁ ˜BL_(n+1) shown in FIG. 5 will be expressed as follows:

    Z(e).sub.0(i,i+1) =Z.sub.0(i+1) [1+Z.sub.0(i+1) ·Z.sub.(i,i+1) +{Z.sub.0(i+1) ·j.sub.(i,i+1) }.sup.2 ]          (3)

    Z(o).sub.0(i,i+1) =Z.sub.0(i+1) [1-Z.sub.0(i+1) ·j.sub.(i,j+1) +{Z.sub.0(i+1) ·j.sub.(i,i+1) }.sup.2 ]          (4)

Using Equations (3) and (4), if the impedances of the even mode and odd mode of the first and last parallel-coupled lines BL₁ and BL_(n+1) in the bandpass filter shown in FIG. 5 are calculated, it is noted that the impedances Z(e)₀(0,1) and Z(o)₀(0,1) of the first parallel-coupled line BL₁ is not the same as the impedances Z(e)₀(n,n+1) and Z(o)₀(n,n+1) of the last parallel-coupled line BL_(n+1). However, for circuit symmetry, when the characteristic impedance Z_(o)(i) of quarter-wavelength input/output lines φL_(2i-1) and φL_(2i) of the ith parallel-coupled line BL_(i) (if it is n/2+1, located in the center of the two-terminal parallel-coupled lines BL₁ ˜BL_(n+1) constituting the bandpass filter of FIG. 5) is set to be the same as the characteristic impedances of adjacent parallel-coupled lines BL_(i-1) and BL_(i+1), the impedances Z(e)₀(0,1) and Z(o)₀(0,1) of tile first two-terminal parallel-coupled line BL₁ are tile same as the impedances Z(e)₀(n,n+1) and Z(o)₀(n,n+1) of (n+1)th parallel-coupled line BL_(n+1). Using the actually obtained impedances Z(e)₀(0,1) ˜Z(e)₀(n,n+1) and Z(o)₀(0,1) ˜Z(o)₀(n,n+1) of two-terminal parallel-coupled lines BL₁ ˜BL_(n+1), if the width and distance of microstrip lines SL₁ ˜SL_(n+2) of two-terminal parallel-coupled lines BL₁ ˜BL_(n+1) are calculated, the values of distances d₁ ˜d_(n+1) between the microstrip lines are all above 0.15 mm, which is easy to accomplish using ordinary print circuit boards. This is because the width and distance of parallel-coupled lines are determined by the impedances of the even mode and odd mode, and the greater the difference thereof is, the wider the distance between parallel-coupled lines becomes. Therefore, in the present invention, the characteristic impedance Z₀(i+1) of line 14 shown in FIG. 6 is set to be greater than Z₀. If the bandpass filter having parallel-coupled lines of FIG. 5 has seven two-terminal parallel-coupled lines when the value of n is six, the widths W_(i) of microstrip lines SL₁ ˜SL₈ constituting seven two-terminal parallel-coupled lines BL₁ ˜BL₇ and distances d_(i) between the microstrip lines are given in the following Table 1.

    <TABLE 1>                                                                      ______________________________________                                         parallel-coupled           distance between                                    line number                                                                              microstrip line width (w.sub.i)                                                                 microstrip lines (d.sub.i)                          ______________________________________                                         BL.sub.1  0.58235 mm       0.1578 mm                                           BL.sub.2  0.89204 mm       0.4743 mm                                           BL.sub.3  0.48427 mm       1.0581 mm                                           BL.sub.4  1.47594 mm       0.5402 mm                                           BL.sub.5  0.48427 mm       1.0581 mm                                           BL.sub.6  0.89204 mm       0.4743 mm                                           BL.sub.7  0.58235 mm       0.1578 mm                                           ______________________________________                                    

Reviewing the width (W_(i)) and distance (d_(i)) of the parallel-coupled lines of Table 1, it is noted that they are symmetric, centering on parallel-coupled line BL₄ and that as the parallel-coupled line numbers increase, the widths of the parallel-coupled lines alternately increase and decrease. Specifically, the width and distance of BL₁ are the same as those of BL₇, those of BL₂ match those of BL₆, and those of BL₃ match those of BL₅. Also, the width of BL₂ is increased more than that of BL₁, the width of BL₃ is decreased more than that of BL₂, and the width of BL₄ is increased more than that of BL₃.

FIG. 7 shows the insertion loss (S21) and return loss (S11) of the bandpass filter having parallel-coupled lines and manufactured according to the values of Table 1. In FIG. 7, the abscissa represents frequency (in gigahertz), and the ordinate represents response (in decibels). The insertion loss at center frequency (14.25GHz) is 2.61 dB, while the return loss is 19.13 dB.

As described above, in the present invention, the distance between parallel-coupled lines is over 0.15 mm so as to provide a bandpass filter having parallel-coupled lines and using a stepped impedance resonator which can be easily manufactured on ordinary print circuit boards. Further, in the present invention, the range of the distance between parallel-coupled lines becomes wider than the conventional one so that the insertion loss of the bandpass filter can be reduced and its bandwidth can be broadened. 

What is claimed is:
 1. A bandpass filter comprising:an input port; an output port; and a plurality of parallel-coupled lines formed between said input and output ports, each of said parallel-coupled lines being formed of segments of a microstrip line having a first portion with a first width and a second portion with a second width, and first portions of adjacent microstrip lines being coupled to form a parallel-coupled line and second portions of adjacent microstrip lines being coupled to form another parallel coupled-line, wherein said each second width is greater than said each first width such that widths of said microstrip lines forming said plurality of parallel-coupled lines alternately increase and decrease, and wherein said microstrip lines forming each parallel-coupled line are separated by a predetermined distance, and said predetermined distance of said microstrip lines forming each parallel-coupled line decreases from parallel-coupled lines located adjacent to a parallel-coupled line centrally located from said input and output ports.
 2. The bandpass filter of claim 1, wherein decreases of said predetermined distance are symmetrical between the parallel-coupled lines disposed between said input port and the centrally located parallel-coupled lines and the parallel-coupled lines disposed between said output port and the centrally located parallel-coupled lines.
 3. The bandpass filter of claim 1, wherein said predetermined distance is at least 0.1 mm. 